Nanosized CuO–ZrxCe1−xOy aerogel catalysts prepared by ethanol supercritical drying for catalytic deep oxidation of benzene

Dry Reforming of Methane over 5%Ni/Ce1-xTixO2 Catalysts Obtained via Synthesis in Supercritical Isopropanol

A series of 5%Ni/Ce_1-xTi_xO₂ catalysts was prepared with nickel impregnation of mixed Ce-Ti oxides obtained via synthesis in supercritical isopropanol. All oxides have a cubic fluorite phase structure. Ti is incorporated into the fluorite structure. Small amounts of impurities of TiO₂ or mixed Ce-Ti oxides appear with Ti introduction. Supported Ni is presented as the NiO or NiTiO₃ perovskite phase. Ti introduction increases total samples reducibility and results in stronger interaction of supported Ni with the oxide support. The fraction of rapidly replaced oxygen and the average tracer diffusion coefficient also increase. The number of metallic nickel sites decreased with increasing Ti content. All catalysts except Ni-CeTi0.45 demonstrate close activity in tests of dry reforming of methane. The lower activity of Ni-CeTi0.45 can be connected to Ni decoration with species of the oxide support. The incorporation of Ti prevents detachment of Ni particles from the surface and their sintering during dry reforming of methane.

Three-dimension titanium phosphate aerogel for selective removal of radioactive strontium(II) from contaminated waters

The effective removal of radioactive strontium (especially ⁹⁰Sr) from nuclear wastewater is crucial to environmental safety. Nevertheless, materials with excellent selectivity in Sr removal remain a challenge since the similarity with alkaline earth metal ions in the liquid phase. In this work, a novel titanium phosphate (TiP) aerogel was investigated for Sr(II) removal from the radioactive wastewater based on the sol-gel method and supercritical drying technique. The TiP aerogel has amorphous, three-dimensional and mesoporous structures with abundant phosphate groups, which was confirmed by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FT-IR). The adsorbent exhibited high efficiency and selectivity for the removal of Sr(II) with an extensive distribution coefficient up to 4740.03 mL/g. The adsorption equilibrium reached within 10 min and the maximum adsorption capacity was 373.6 mg/g at pH 5. And the kinetics and thermodynamics data fitted well with the pseudo-second-order model and Langmuir model respectively. It can be attributed to the rapid trapping and slow intraparticle diffusion of Sr(II) inside the mesoporous channels of the TiP aerogel. Furthermore, TiP aerogel exhibited over 80% removal for 50 mg/L Sr²⁺ in real water systems (seawater, lake water and tap water). X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed that strong ionic bonding formed during Sr(II) adsorption with the phosphate group on TiP aerogel. These results indicated that TiP aerogel is a promising high-capacity adsorbent for the effective and selective capture of Sr(II) from radioactive wastewater.

Nickel-Containing Ceria-Zirconia Doped with Ti and Nb. Effect of Support Composition and Preparation Method on Catalytic Activity in Methane Dry Reforming

Nickel-containing mixed ceria-zirconia oxides also doped by Nb and Ti have been prepared by a citrate route and by original solvothermal continuous flow synthesis in supercritical alcohols. Nickel was subsequently deposited by conventional insipient wetness impregnation. The oxides are comprised of ceria-zirconia solid solution with cubic fluorite phase. Negligible amounts of impurities of zirconia are observed for samples prepared by citrate route and doped by Ti. Supports prepared by supercritical synthesis are single-phased. XRD data, Raman, and UV-Vis DR (diffuse reflectance) spectroscopy suggest increasing lattice parameter and amount of oxygen vacancies in fluorite structure after Nb and Ti incorporation despite of the preparation method. These structural changes correlate with the catalytic activity in a methane dry reforming reaction. Catalysts synthesized under supercritical conditions are more active than the catalysts of the same composition prepared by the citrate route. The catalytic activity of samples doped with Ti and Nb is two times higher in terms of TOF (turnover frequency) and increased stability of these catalysts is attributed with the highest oxygen mobility being crucial for gasification of coke precursors.

Synthesis of aerogels: from molecular routes to 3-dimensional nanoparticle assembly

Colloidal nanocrystals are extensively used as building blocks in nanoscience, and amazing results have been achieved in assembling them into ordered, close-packed structures. But in spite of great efforts, the size of these structures is typically restricted to a few micrometers, and it is very hard to extend them into the macroscopic world. In comparison, aerogels are macroscopic materials, highly porous, disordered, ultralight and with immense surface areas. With these distinctive characteristics, they are entirely contrary to common nanoparticle assemblies such as superlattices or nanocrystal solids, and therefore cover a different range of applications. While aerogels are traditionally synthesized by molecular routes based on aqueous sol-gel chemistry, in the last few years the gelation of nanoparticle dispersions became a viable alternative to improve the crystallinity and to widen the structural, morphological and compositional complexity of aerogels. In this Review, the different approaches to inorganic non-siliceous and non-carbon aerogels are addressed. We start our discussion with wet chemical routes involving molecular precursors, followed by processing methods using nanoparticles as building blocks. A unique feature of many of these routes is the fact that a macroscopic, often monolithic body is produced by pure self-assembly of nanosized colloids without the need for any templates.

Effects of four CeO2 nanocrystalline catalysts on early-life stages of zebrafish Danio rerio and crustacean Daphnia magna

Effects of four different nanocrystalline CeO(2)-based catalysts on crustaceans Daphnia magna and early-life stages of zebrafish Danio rerio were studied. Pure CeO(2) and CuO-CeO(2) mixed oxides with a nominal 10, 15 and 20 mol.% CuO content were tested. Pure CeO(2) provoked no effects, but CuO-CeO(2) mixed oxides induced some sublethal effects on fish and affected daphnids' survival. The most pronounced effects were found on fish body growth, which was reduced at 10 mg/L in case of CuCe20 and 50 mg/L in cases of CuCe10 and CuCe15. Daphnids' survival was affected above 80 mg/L of CuCe20, while CuCe10 and CuCe15 did not affect daphnids. None of the materials was highly toxic to daphnids and fish in comparison to some other environmental pollutants. Differences in effects between the materials could not be explained by their specific physicochemical properties. This work indicates that more attention should be placed at potential toxicity of nanostructured materials, such as nanocrystalline mixed-oxides.

Aerogel Catalysts

Aerogels are often largely mesoporous solids, with a porosity which may exceed 90 vol% and a specific surface area up to 1000 m2 g-1. Such materials were first obtained by Kistler in 1932, and designate gels in which the liquid was replaced with a gas without collapsing the gel solid network. Contrary to xerogels dried from wet gels by evaporation with an important shrinkage, the first aerogels were obtained by a “supercritical drying” technique in which the liquid which impregnated the gels was evacuated after being transformed to a supercritical fluid. The diversity in nature of the solid constituting the rigid network is very large. It includes simple oxides, multi oxide compositions, organic and hybrid organic-inorganic polymers and carbon. This diversity as well as the high specific pore volume and surface area make aerogels applicable either as catalysts or as catalyst supports. Besides, molecular catalysts such as transition metal complexes or enzymes can easily be immobilized in aerogels, which opened the road to new supported molecular catalysts and biocatalysts. This communication reviews the synthesis and properties of oxide aerogel catalysts.